a) Field of the Invention
This invention relates to a viscous damper for a hydraulic power transmission such as a torque converter which is used in an automatic transmission for a vehicle and is equipped with a lockup clutch. The term "viscous damper" means a vibration damping device making use of viscous resistance.
b) Description of the Related Art
A torque converter which is recently used in a power transmission or the like arranged in association with an engine of an automotive vehicle is a kind of hydraulic power transmission using a pump and a turbine. Even when the turbine and the pump become substantially equal in revolution speed, there is still a relative sliding motion between the pump and the turbine so that the torque converter has lower mechanical efficiency of power transmission compared with a straight-line transmission mechanism making use of a mechanical clutch.
In a space formed between an input casing fixed on an input shaft of the torque converter, to which input shaft power is transmitted from the engine, and outer peripheral faces of turbine runners, a lockup clutch capable of directly connecting the input shaft and the output shaft with each other is therefore often arranged.
With reference to FIG. 1 of the accompanying drawings, one example of the construction of a torque converter 10 with a lockup clutch will be described first. Illustrated in FIG. 1 are an input casing 1 fixed on an input shaft (not shown) extending from an unillustrated engine, a pump 2 fixed on the input casing 1, a turbine 3 located opposite the pump 2, a stator 4, a first driven plate 5 connected with the turbine 3 and a power transmitting portion 7 by a connector 11, a second driven plate 6 fixedly secured to the first driven plate 5 by a rivet 9, an output shaft 8 connected to the power-transmitting portion 7 via a spline engagement or the like, a drive plate 21, a clutch piston 23 of the lockup clutch, said clutch piston 23 being slidably disposed in a space between the input casing 1 and the turbine 3, and a friction lining 24 arranged on the clutch piston 23. The drive plate 21 is coupled at an engaged portion 22 thereof with the clutch piston 23. Further, power can be transmitted between the drive plate 21 and the first and second plates 5,6 while permitting a slight relative motion therebetween via a spring 25.
When the torque converter 10 is operated without using the lockup clutch, power from the engine enters the input casing 1 and hence rotates the pump 2 so that the turbine 3 located opposite the pump 2 is rotated. As described above, the turbine 3 is fixed on the power transmitting portion 7 by the connector 11 and the power transmitting portion 7 is connected to the output shaft 8. Rotation of the turbine 3 is therefore transmitted to the output shaft 8.
When the lockup clutch is in operation, the clutch piston 23 is pushed leftward by a hydraulic pressure as viewed in the drawing so that the clutch piston 23 is rotated integrally with the input casing 1 with the friction lining 24 interposed therebetween. An input from the engine is therefore transmitted directly to the clutch piston 23 via the input casing 1.
As the drive plate 21 is coupled at the engaged portion 22 thereof with the clutch piston 23, the drive plate 21 rotates integrally with the clutch piston 23 and the rotation of the drive plate 21 is transmitted to the first and second driven plates 5,6 via the spring 25. Because the first driven plate 5 is fixed on the power transmitting portion 7 by the connector 11, the power from the engine is accordingly transmitted to the output shaft 8 via the lockup clutch.
Power is transmitted between the drive plate and the driven plates via the spring in the conventional torque converter with the lockup clutch. Owing to damping action of the spring, it is therefore possible to absorb and reduce a torsional vibration or an impact even if such a torsional vibration or impact is produced on the input shaft.
The conventional torque converter with the lockup clutch can therefore damp or reduce a single torsional vibration or impact of the shaft produced by a large fluctuation in torque, but is accompanied by the drawback that it can hardly control self-excited minute vibrations which are continuously produced in the course of engagement of the lockup clutch.